Detachment of Oil Drops from Solid Surfaces in Surfactant Solutions : Molecular Mechanisms at a Moving Contact Lin

1. Detachment of Oil Drops from Solid Surfaces in Surfactant Solutions: Molecular Mechanisms at a Moving Contact Line Paper Submitted to: Industrial & Engineering Chemistry Research by Peter A. Kralchevsky, Krassimir D. Danov, Vesselin L. Kolev, Theodor D. Gurkov, Mila I. Temelska, and Günter Brenn Drop detachment: Moving contact line; Importance for Detergency; Membrane emulsification; Other processes with moving three-phase contact line Aim: (i) Examine the effect of temperature, surfactant and salt concentrations, on the dynamics of drop detachment. (ii) Develop of a quantitative theoretical model; fitthe experimental data; determine the values of the involved physicochemical parameters.

2. Experiment Scheme of the experimental cell:1 – glass plate; 2 – oil droplet  1 l; 3 – glass holders;4 – surfactant solution; 5 – syringe; 6 – cuvette. Procedure: (1) Oil drop is placed on the dry glass substrate; (2) Solution of surfactant + NaCl is poured in the cuvette; (3) The process of oil drop detachment is recorded by horizontal microscope and video-camera.

3. Dynamics of Spontaneous Drop Detachment 0 s 27 s 1 min 55 s 9 min 16 s 21 min 49 s 1 h 6 min 46 s 1 h 30 min 55 s 2 h 5 min 55 s 2 h 12 min 20 s

4.  2rc 2rc  Drop Profile: Digitization and Fit by Laplace Equation Results from the fit: Determination of thecontact radius, rc,and contact angle, ,as functions of time, t.

5. Effect of SDS concentration on detachment of hexadecane drops from glass: Data for the contact radius, rc, and for the contact angle, , plotted vs. time; the initial moment, tin, corresponds to the first experimental point. The NaCl concentration is 0.1mM and the temperature is 23 C. The increase of the SDS (sodium dodecyl sulfate) concentration accelerates the oil-drop detachment.

6. Effect of AOS concentration on detachment of hexadecane drops from glass: Data for the contact radius, rc, and for the contact angle, , plotted vs. time; the initial moment, tin, corresponds to the first experimental point. The NaCl concentration is 100mM and the temperature is 23 C. The increase of the AOS (sodium C16 alpha-olefin-sulfonate) concentration accelerates the oil-drop detachment.

7. Effect of NaCl concentration on detachment of hexadecane drops from glass: Data for the contact radius, rc, and for the contact angle, , plotted vs. time; the initial moment, tin, corresponds to the first experimental point. The AOS concentration is 6 mM and the temperature is 23 C. The increase of the NaCl concentration accelerates the oil-drop detachment.

8. Effect of temperature on detachment of hexadecane drops from glass: Data for the contact radius, rc, and for the contact angle, , plotted vs. time; the initial moment, tin, corresponds to the first experimental point. The AOS concentration is 6 mM and the NaCl concentration is 316 mM. The increase of the temperature accelerates the oil-drop detachment.

9. Theoretical Basis At equilibrium: the Young equation holds During relaxation: the Young equation contains an additional friction term, which compensates the imbalance of the tree interfacial tensions:  is the line friction coefficient

10. Determination of the Line Friction Coefficient Set of preliminary data: glass plates cleaned by sulfo-chromic acid From the slope of the best fit we determine the line friction coefficient:  = 1.6 Pa.s Basic question: How does  depend on surfactant and salt concentrations and on the temperature?

11. New Set of Data The glass plates are used as provided by the producer; no cleaning by sulfo-chromic acid The difference osws is not constant, but varies with time: Consequence of the formation of a gel layer on the glass surface in contact with water.

12. Data Interpretation: Diffusion of water into the surface layer of glass and development of a gel layer Theoretical model is developed, which accounts for the penetration and diffusion of water in the surface of glass, and for the dependence of osandws on the concentration of water in the gel layer at the contact line.

13. Theoretical Model Diffusion equation for region 2 Concentrations and fluxes equal at the boundary region 1 / region 2 Limiting case of fast diffusion: Diffusion equation for region 1(the last term accounts for the influx from the water phase); D – diffusivity of water in the gel layer; c – concentration of water in the gel layer; tp – characteristic penetration time. cb – concentration of water at the boundary between regions 1 and 2 (at r = rc); ceq – equilibrium value of c.

14. Comparison of Theory and Experiment We assume a simple Henry law for the interfacial tensions: Integrated equation of contact-line motion: (t) is known from the experiment; adjustable parameters: , , tp and tin. Excellent agreement between theory and experiment is obtained!

15. Effect of Surfactant on Line Friction Coefficient and Penetration Time The detected effect of surfactanton andtp is most probably related to its role for hydrophilization and/or removal of the hydrophobic coverage (gloss)of the glass surface.

16. Effects of Salt and Temperature The salt facilitates the surfactant adsorption. The temperature is known to reduce the viscous effects and accelerate the diffusion processes.

17. Conclusions 1. The increase of temperature, surfactant and salt concentrations accelerate the oil-drop detachment. 2. The spontaneous drop detachment is due to penetration of water in a thin (gel) layer at the surface of glass. 3. The data are excellently fitted by the dynamic Young equation, and the line friction coefficient, , is determined.